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Chemistry quiz, Is your chemistry knowledge as light as helium or as heavy as plutonium?

The periodic table is a list of all known elements — everything from oxygen to neon and einsteinium to uranium. For such an important table, it's a patchy looking thing. But the quirky layout isn't because science couldn't afford a decent graphics guy — it's set out that way to show us the two most important things about the atoms of any element:

how many protons are in the nucleus, and

how many electrons are buzzing around in the outermost shell.

They may not sound like much, but a single proton and one electron are the only difference between the carbon atoms in a diamond and the nitrogen atoms that make up 80 per cent of the air we breathe.

Protons decide which element an atom is, while outermost electrons call the shots on how the atom behaves chemically. And between them, they determine which elements can conduct heat and electricity, which are gases, and which are capable of being the basis of life.

And you can tell all of that and more just by knowing where an element sits on the periodic table. It's not exactly the Da Vinci code, but there's an underlying message in the rows and columns of the table.

If you don't know anything about how electrons are arranged around an atom, the pattern in the rows won't make any sense. But if you've got even a distant memory of Year 9 chemistry and electron shells, some bells might start ringing.

Rows and electron shells

Electrons aren't buzzing around the nucleus of an atom like flies around manure — they exist in different energy 'shells'. Each shell can only hold a certain number of electrons, and the shells have to be filled in order from the one closest to the nucleus out.

Starting from the shell closest to the nucleus, the electron shells of an atom can hold two, eight, eight, 18, 18, 32 and 32 electrons. And in order from top to bottom, the rows of the periodic table hold two, eight, eight, 18, 18, 32 and 32 elements. So when you see the position of an element in the table, you can tell straight away how full its outermost electron shell is.

Hydrogen in row 1, column 1 has one electron in shell 1. Sodium (Na, row 3, column 1) has one electron in its third shell. Magnesium (Mg) has two outer electrons, oxygen (O) has 6, and carbon (C) and silicon (Si) both have four. And in the far right column are the six known elements with a full outer shell, the noble gases.

And for atoms, a full outer shell is where it's at. Atoms with a full outer shell are chemically stable — they can sit around unchanged for the rest of eternity (or at least until they get swallowed by the sun or sucked into a black hole and fused together with other atoms). But while only noble gases are 'born' that way, all the other elements can fake their way to a more stable existence. Their atoms can share, steal or offload electrons by forming chemical bonds with other atoms so they can each end up with a full outer shell.

And that's where the table's layout really comes into its own. Like a match-making guide for elements that aren't exactly noble, at a glance you can tell what goes with what, and how many times.

Shells?Okay, okay! Electrons aren't really in shells; they're in probability clouds with different energy levels. But the great thing about the shells is that those of us who might struggle with grasping quantum behaviour (like collapsing probability clouds) can get our heads around shells, and for the basics of chemistry the shell model plus table is plenty good enough.

Columns and match-making, salt and water

Atoms looking to partner up with other atoms to score a full outer electron shell just have to follow a few periodic 'dating' rules.

Everything on the left of the table (metals, in blue) has got to lose electrons.

Everything on the right (non-metals, in green) has got to gain electrons.

Non-metals can gain electrons by stealing them from metals (ionic bonding, like salt) or by sharing some of theirs with some of another atom's electrons (covalent bonding, like water).

And the reason for the rules comes down to the how much electron pulling power your protons have got.

The only thing that keeps an atom's electrons from flying off into space is the attractive force from the protons. Protons are positive, electrons negative — it's classic boy meets girl meets subatomic theory stuff.

Every electron is held in its place by the combined pulling power of all the protons in the nucleus. But the further away from the nucleus electrons get, the less effective the attractive force from the protons there becomes.

Sodium (Na) has eleven protons and eleven electrons. The electrons all feel the pull of 11 protons, but it's strongest for the inner two, then a bit less strong for the eight in the second shell. The lone electron in the outer shell is so far away from the nucleus it barely feels a tug.

Over in column VII of the same row, chlorine (Cl) has got 17 protons and 17 electrons. The two electrons in its first shell are held even more strongly than those in sodium's inner shell because they feel the effect of 17 protons all up. The second shell electrons are a bit closer to the nucleus than those in sodium for the same reason. And those 17 protons have got a very firm hold on the seven outer-shell electrons. There's no way an outer electron can escape its chlorine atom (short of being poked by the equivalent of an atomic cattle prod). In fact, their pulling power is easily enough for chlorine's protons to pinch and hold onto a stray outer electron from sodium.

Most elements have outer electrons that are as weakly bound as those of sodium. We call them all metals. Not because they'd make great frying pans or car parts, because the loose electrons makes them great conductors of heat and electricity.

But non-metals don't have to steal electrons to fill their outer shells. Unlike metals they're able to share electrons with each other, thanks to all those protons in their nuclei, and create a less-is-more outer shell.

Oxygen exists as molecules of O2, two oxygen atoms each pooling two of their outer electrons into a share arrangement, so each atom has got eight outer electrons in its outer shell. So much more stable than the single life …

But oxygen can branch out beyond its own kind, which is lucky for us because without water — oxygen sharing electrons with hydrogen — life on this and every other planet couldn't exist.

Water isn't the only element necessary for life. All life forms we know about are based on carbon, and that's because of where it sits in the periodic table.

The fourth element in the second row, carbon's outer shell has four of a possible eight electrons — it's either half-empty or half-full depending on your disposition. To fill that shell, a carbon atom has got to share electrons with up to four other atoms. If it binds with a carbon atom, that carbon can bind with three other carbons. And its that ability to form molecules that can have branching chains that makes carbon the ideal element for building complex molecules like proteins, fats and carbohydrates that life is based on.

The only other element that can build complex molecules is silicon, right below carbon in column IV with its own four outer electrons to share. We haven't found any life forms based on silicon (outside Star Trek episodes), but silicon polymers have transformed everything from baking dishes to bathing caps.

Columns, rows, size and pulling power

Elements in the same column have got the same number of electrons in their outer shells, so they react in similar ways chemically.

As you go across a row, elements have more protons in their nucleus, so their electrons are held more firmly. That improved electron pulling power (electronegativity in science lingo) means elements get less metallic as you go across a row. It also means they get smaller (lower atomic radius), because with each additional proton the electrons are more closely held.

The opposite happens as you go down a column — the atoms further down have more protons pulling on their electrons, but they've also got more shells of electrons, so the atoms are a bit heftier (greater atomic radius). And even the most charismatic bunch of protons can't attract passing electrons over that great a girth (lower electronegativity).

Throw in a secret sect and a French heiress and you've got the makings of some Dan Brownian motion.